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Show 216 GEOLOGY OF THE HIGH PLATEAUS. we may note the stones which pave its bed, and after a flood has passed and the stream again is clear we may find that there has been little change in them; but to conclude that no stones have passed in the interval would be a mistake. Those which retain their places have lodged there and been fastened to the bottom by a packing of sand or wedged together like the cobbles of a pavement. If the sources of the materials continue to furnish them, doubtless many stones have been hurried along over this pavement during the flood, a few finding a resting place, but more of them passing on to be ground into silt or to find resting-places in deeper waters below. But there is another method quite different from this precipitate one, and by which it is very probable that much larger movements are effected, though much more slowly. It never happens that the materials to be moved are of uniform grain. Mud, sand, gravel, shingle, and cobblestones always accompany coarser debris in varying proportions, and form a matrix in which the larger fragments are imbedded. An acceleration of the current removes the finer stuff and retardation replaces it with fresh. The washing out of the matrix of sand and grit which holds a pebble in its place leaves the pebble to the unobstructed energy of the current. If that energy is sufficient it will be carried along until the current slackens or until it finds a lodgment. If the energy is too small, the pebble will remain until the ceaseless wear of attrition reduces it and brings it within the power of the stream to move it. Nor are these movements dependent solely upon periodical floods. Any cause wdiich alternately accelerates the movement of water may produce them, and these causes are many. Every stream and every shore current is affected by numerous rhythmical movements which produce these alternations in many ways and many degrees. The waves and surf, the undertow7, the tides, the shifting of shore currents, the storms and monsoons, the ripples of the brook, the numberless surgings and waverings of rivers, the shifting of channels, the building and destruction of sand bars, the freshetsâ€"all are causes by virtue of which any spot at the bottom of the water is subject to alternate maxima and minima in the velocity of the water which passes over it. Sooner or later, then, the pebble must move on, provided any maximum of velocity in |